An organic EL device in the simplest structure is generally constituted of a light-emitting layer and a pair of counter electrodes sandwiching the said light-emitting layer. The device functions by utilizing the following phenomenon; upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and the excited state created by recombination of the electrons and holes in the light-emitting layer returns to the ground state with emission of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been directed toward commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer composed of an aromatic amine and a light-emitting layer composed of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state, is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer was investigated, but these derivatives merely produced luminance at an extremely low level. Europium complexes were also investigated in trials to utilize the excited triplet state, but they too failed to emit light at high efficiency.    Patent document 1: JP2003-515897 A    Patent document 2: JP2001-313178 A    Patent document 3: JP2002-305083 A    Patent document 4: JP2003-142264 A    Patent document 5: JP 11-162650 A    Patent document 6: JP 11-176578 A    Non-patent document 1: APPLIED PHYSICS LETTERS, Vol. 75(1), pp. 4-6, 1999    Non-patent document 2: APPLIED PHYSICS LETTERS, Vol. 78(11), pp. 1622-1624, 2001    Non-patent document 3: APPLIED PHYSICS LETTERS, Vol. 89, p. 061111-1-3, 2006
A large number of phosphorescent dopants useful for the light-emitting layer of an organic EL device are disclosed in patent document 1 and elsewhere. A typical example is tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3).
A substance proposed as a host material for the light-emitting layer of an organic EL device is 4,4′-bis(N-carbazolyl)biphenyl (hereinafter referred to as CBP), a carbazole compound cited in non-patent documents 1 and 2 and patent document 2 and elsewhere. From the viewpoint of triplet exciton confinement, CBP is used widely as a host material for Ir(ppy)3, a phosphorescent material emitting green light, and for octaethylporphyrin platinum complex (hereinafter referred to as PtOEP), a phosphorescent material emitting red light.
However, an organic EL device comprising CBP as a host material in the light-emitting layer has faced a problem that the barrier to injection of holes from the hole-transporting layer composed of an arylamine compound into the light-emitting layer tends to become higher and the driving voltage becomes higher.
On the other hand, CBP has a property of allowing holes to flow more easily than electrons; hence, excess holes tend to flow out to the side of the electron-transporting layer when CBP is used and this is one cause for lowering of the luminous efficiency. As a means to solve this problem, a hole-blocking layer is provided between the light-emitting layer and the electron-transporting layer as done, for example, in patent document 3. The hole-blocking layer thus provided accumulates holes efficiently in the light-emitting layer and improves the probability of recombination of holes and electrons in the light-emitting layer thereby achieving the object of enhancing the luminous efficiency. The hole-blocking materials in general use at present include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1,O8)aluminum (hereinafter referred to as BAlq). These materials are capable of preventing electrons and holes from recombining in the electron-transporting layer. However, BCP lacks reliability as a hole-blocking material on account of its tendency to crystallize easily even at room temperature and a device containing BCP shows an extremely short lifetime. On the other hand, BAlq has a Tg of approximately 100° C. and a device containing it is reported to show a relatively long lifetime; however, the hole-blocking ability of BAlq is not sufficient and the luminous efficiency from Ir(ppy)3 becomes lower.
Further, it is reported in patent document 4 that BAlq is applicable as a host material for a light-emitting layer comprising a phosphorescent material emitting red light. The use of BAlq as a host material produces an effect of attaining high efficiency with no need to provide a hole-blocking layer between the light-emitting layer and the electron-transporting layer. Thus, the device would be freed from the unstable factors attributable to the hole-blocking material and might be expected to improve in the lifetime. However, BAlq has a property of hindering the flow of holes and, besides, the light-emitting layer must be made relatively thick when a hole-blocking layer is omitted and the problem here is a rise in the driving voltage.
Still further, it is reported in non-patent document 3 that 4,4′-bis[(N-1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB), a material in widespread use as a hole-transporting material, is applicable as a host material for a light-emitting layer comprising a phosphorescent material emitting red light. The use of NPB as a host material in the light-emitting layer produces an effect of eliminating the barrier to injection of holes from the hole-transporting layer into the light-emitting layer and holding the driving voltage at a low level. However, the problem with arylamine compounds, typically NPB, has been that the substituents attached to their nitrogen atom tend to move easily due to the molecular structure of these compounds thereby increasing the probability of nonradiative deactivation of triplet excitons and, as a result, the efficiency becomes lower. According to non-patent document 3, an attempt is made to enhance the luminous efficiency by adding an electron-absorbing material to the hole-injecting layer and an electron-donating material to the electron-transporting layer and raising the density of holes and electrons in the light-emitting layer.
Further, the indolocarbazole compounds disclosed in patent documents 5 and 6 are recommended for use as a hole-transporting material and are reputed to be stable. However, the documents do not teach their use as a phosphorescent host material.